1. Field of the Invention
The present invention relates to a sheet feeder that separates and conveys stacked sheets (such as recording sheets or OHP films) one by one by the rotation of a sheet feed roller. These stacked sheets may be used in image forming devices, such as copying machines, printers, and facsimiles. More specifically, the invention relates to a sheet feeder capable of suppressing an excessive torque applied to a sheet feed roller, thus preventing damages to the sheet feed roller or a driving unit for the sheet feed roller. Such an excessive torque is generated when there is no stacked sheet present in the sheet feeder. As used herein, a roller or a feeder that conveys objects other than sheets of paper are called a sheet feed roller or a sheet feeder respectively.
2. Discussion of Related Art
A sheet feeder used for image forming devices, such as copying machines, printers, and facsimiles, that is configured such that stacked sheets are separated and conveyed one by one is known in the related art. This type of sheet feeder is equipped with (1) a placing plate on which sheets are stacked, (2) a sheet feed roller that is arranged so as to face the placing plate, and (3) a friction member that is disposed in a position on the placing plate facing the sheet feed roller.
According to this sheet feeder, a biasing force of a biasing means places the sheet feed roller in pressure contact with a stack of sheets on the placing plate. As the sheet feed roller rotates in a predetermined direction, only the uppermost sheet of the sheets stacked on the placing plate is separated from the remaining sheets and conveyed in a predetermined conveying direction. At this time, since a frictional force generated between the roller surface of the sheet feed roller and the uppermost sheet is greater than a frictional force generated between the uppermost sheet and a sheet underneath the uppermost sheet, sheets are separated one by one, and conveyed in a predetermined conveying direction.
Further, the lowermost sheet abuts the friction member, and thus the lowermost sheet is suppressed in its movement by a frictional force generated between the friction member and the lowermost sheet. For this reason, even when a few sheets are left on the placing plate, the so-called double feeding, where the lowermost sheet is conveyed together with the sheet above the lowermost sheet, is prevented. As a result, the stacked sheets are separated one by one and conveyed in a predetermined conveying direction. Also, when there is no sheet on the placing plate, the biasing force of the biasing means brings the sheet feed roller into contact with the friction member.
Meanwhile, when the sheet feed roller is caused to rotate in the state where the sheet feed roller is in contact with the friction member, an excessive frictional force is generated between the friction member and the surface of the sheet feed roller. Therefore, an excessive torque is applied to the sheet feed roller. Accordingly, unpleasant noises may be generated, or the sheet feeder itself may malfunction.
Thus, in order to avoid applying an excessive torque to the sheet feed roller, JP-A-8-259013 discloses a sheet feeder configured such that a freely rotatable roller that is connected to a spring is provided in a portion of a bottom plate that faces a sheet feed roller, and when there is no sheet on a placing plate, the roller surface of the sheet feed roller is caused to abut the peripheral surface of the freely rotatable roller so that an undesired frictional force cannot be generated.
However, in such a configuration, double feeding may occur when there are only a few sheets on the placing plate (e.g., two sheets, three sheets, etc.). When there is a great enough number of sheets (e.g., several tens of sheets) on the placing plate, such that the weight of the sheets is sufficient, the freely rotatable roller is suppressed in its rotation by the weight of the sheets and the biasing force of the spring. Therefore, when there is a sufficient number of sheets on the placing plate, the uppermost sheet can be separated and conveyed individually. However, when there are only a few sheets on the placing plate, the force against the rotation of the freely rotatable roller becomes too weak to suppress the rotation of the freely rotatable roller, resulting in double feeding.
In addition, the frictional force between the sheet feed roller and the uppermost sheet may be reduced due to adhesion of foreign substances, such as paper debris, to the surface of the sheet feed roller. Accordingly, the supply of recording sheets may become unstable.
The present invention has been made in consideration of the above problems. It is therefore an object of the invention to provide a sheet feeder and an image forming device that are (1) capable of preventing an excessive load from being applied to a sheet feed roller when there is no sheet on the placing surface, (2) capable of preventing double feeding of sheets, and (3) capable of restoring frictional force on the surface of the sheet feed roller.